Final answer:
The unilateral movement of the sacrum is called lateral flexion, which is the side-bending of the body or neck.
Step-by-step explanation:
When the sacrum moves unilaterally, it participates in the action known as lateral flexion. Lateral flexion is the bending of the neck or body towards the right or left side, which occurs within the coronal plane. The sacroiliac joints allow for this kind of movement, albeit in a very limited range. This movement is not to be confused with rotation, flexion, or extension, which are different types of vertebral column movements.